This invention was made during the course of, or under, a contract with ERDA.
This invention relates generally to electronic chopper circuits and more specifically to an improved Darlington transistor type "up-and-down" chopper circuit configuration provided with additional circuit elements for eliminating the effects of inter-electrode capacitance inherent with these transistors.
Until recently most inverters have used SCR's as switching elements because they were the only devices available with the necessary high voltage and current ratings. Darlington transistors now offer the high voltage and current ratings needed to provide an alternative in the design of power stage of inverter circuits. Use of Darlington produces a power stage which is significantly smaller in size and weight and is also less costly because of the absence of commutation components. The chopper is connected between a DC source and a load which stores energy. Examples of loads that store energy are: (1) DC motors; (2) DC to AC inverters driving AC motors; and, (3) battery banks in DC link systems.
In the down mode the DC power source provides power to drive the load. The chopper circuitry controls load power consumption by controlling the load voltage. Load voltage is controlled by reducing the voltage available from the DC source, hence the term "down chopper."
The up mode of operation is the regenerative mode. In this mode the load acts as a power source. If the load is a motor, the up mode is the regenerative motor breaking mode. The motor acts as a generator with the inertial energy stored in the motor and its mechanical load providing the motor power. In the regenerative mode, the voltage produced at the load terminals can be less than the DC power source voltage. The chopper circuitry generates an increased voltage from the low voltage, hence the term "up chopper"; this permits the return of power to the DC power source during the regenerative mode.
In the conventional chopper circuit, employing regenerative operation, a pair of solid state switches are employed, such as Darlington transistors, in a series-shunt relationship between the source and the load. A smoothing inductor is connected in series with the load and filters the chopped current produced by the "down" or "up" chopper portion of the circuit. This provides direct current, rather than pulsed current, to or from the load. The transistors are shunted by free-wheeling diodes to provide proper current flow in the circuit during chopper operation.
In a circuit configuration of this type, using Darlington transistors, a design problem exists in that these transistors turn "on" when a large step change of voltage (high dv/dt) is applied across the collector and emitter. This phenomenon, which occurs in the absence of base drive, is due to inter-element capacitance.
In the down mode, when the series transistor is rapidly switched "on" and "off", the collector of the shunt transistor is subjected to a positive step change of voltage each time the series transistor is turned "on". This causes the shunt transistor to turn "on" for a short period of time, creating a short circuit across the power source.
During the regenerative mode the shunt transistor is rapidly switched "on" and "off". Thus a step voltage is applied to the series transistor each time the shunt transistor is turned "on". This causes the series transistor to turn "on" for a short period of time, creating a short across the power source.
The unique solution to this problem by means of circuit additions which prevent the unwanted turn on of the shunt transistor in the down mode and the unwanted turn on of the series transistor during the regenerative mode, is the subject of this invention.